[146224] in North American Network Operators' Group
Re: mtu question. more should be better, right?
daemon@ATHENA.MIT.EDU (Iljitsch van Beijnum)
Mon Nov 7 12:02:47 2011
From: Iljitsch van Beijnum <iljitsch@muada.com>
In-Reply-To: <CAAiP322G=5AoEcu46NPOqOC3-wPoyXP5-dth5_YSTbO_vhAqoQ@mail.gmail.com>
Date: Mon, 7 Nov 2011 18:01:40 +0100
To: Deric Kwok <deric.kwok2000@gmail.com>
Cc: nanog list <nanog@nanog.org>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
On 7 Nov 2011, at 17:45 , Deric Kwok wrote:
> When I setup the server mtu as 9100. why I have to configure the
> switch mtu 9300 to make it working?
> What this extra 200 bytes is for what purpose? ls it standard?
To avoid problems you really want to set the MTU of all your IP devices =
on the same subnet to the same value. On the switches the MTU needs to =
be big enough to accommodate the MTU that the hosts and the routers use, =
but there's no harm in it being larger. (Although you may be using up =
memory for nothing.)
One complication is that not everyone has the same understanding of =
packet sizes. For IP the MTU is the size of the largest IP packet that =
can be carried, but layer 2 people sometimes add 14 or 18 bytes to that =
and layer 1 people maybe even 38. (14 =3D ethernet header, 18 =3D =
ethernet header + frame check sequence, 38 =3D header, FCS, preamble, =
start of frame delimiter and interframe gap.)
> What is disadvantage of setting our all internal networks (host /
> equipment) mtu more than 1500?
Mostly that you'll be hitting path MTU discovery more often. However, =
this will not cause many issues unlike the case where you use an MTU =
_smaller_ than 1500. Don't set a larger MTU on slow networks (certainly =
not on 10 Mbps) because long packets sit in queues for a long time at =
low speeds, getting in the way of interactive traffic such as VoIP. =
Above 11k the ethernet frame check sequence catches fewer errors.=
